The invention is related to a method for measuring instantaneous electrical parameters of power distribution systems utilizing digital control means and to a programable control apparatus performing such method.
The power source of a typical distribution system is a generator producing a large quantity of power at medium voltage level. The power is stepped up by a transformer and sent over a transmission line. At the other end of the transmission line a step-down transformer is provided for transforming the power down to a sub A transmission level for distributing it to widely scattered distribution substations. In the individual substation the power is again stepped down and fed out onto distribution feeders which include distribution transformers for transforming the feeder power to 120/240 volts eventually supplied to individual customers.
The entire function of such a distribution system is to deliver power to the end user at a voltage within acceptable limits; this is easily understood when considering that all electrical appliances are designed for optimum performance if supplied by power of a specific voltage level and a fixed frequency.
The frequency is here of less concern, since it can be kept a constant solely by regulating the power source, whereas it is one of the basic constraints of a power distribution system to supply a rated voltage to each individual customer. Within the system, measurements are taken for evaluating instantaneous load conditions and for adjusting accordingly the operation characteristics of the distribution system in order to supply such rated voltage to the individual customer independent from changing load conditions of the system.
Large distribution systems do not solely rely upon generator regulators for such purpose, more often additional measures are taken for correcting the voltage at various locations throughout the system. A variety of measures are conventionally utilized for such voltage regulation controlling the output voltage at specific points of the distribution system based upon the current line resistances and reactances and the instantaneous load condition. Voltage regulators are primarily used at sub-transmission level or on distribution feeders between the substations and the consumer.
Merely for the purpose of illustration, a well known type of voltage regulator, the single- or three-phase auto-transformer with voltage regulating tabs in one of the windings and including a motor driven tab-changing mechanism may be mentioned. The auto-transformer basically is a transformer in which primary and secondary windings are coupled both mechanically and electrically for producing a regulated step-up or step-down voltage. Since sudden boosts in voltage on a circuit would have adverse effects, a fine regulation of conventionally 32 voltage steps within a range from 10% below to 10% above normal voltage are provided. Since a manually controlled regulator would have little utility, it is imperative to have the tab position of the regulator controlled automatically by a control device that is sensitive to the output voltage of the regulator. For this reason, the regulator adjustment mechanism typically includes self-positioning, spring driven contacts loosely coupled to a driving motor or electrically positioned contacts directly geared to this motor. The operation of the motor drive is controlled by a control unit that evaluates the output characteristics of the regulator in order to raise or lower the same to a predetermined level.
Conventionally, such control units are designed as analog electronic devices which include electrical circuits for sensing the output voltage of the regulator, for compensating a line voltage drop, and for providing a time delay function. The voltage censing circuit is connected to the output circuit of the auto-transformer through a scaling potential transformer and evaluates the instantaneous output voltage with respect to a predetermined voltage in order to derive an output signal governing the motor of the tab changing mechamism. Since regulators are usually located remotely from the load center, even a constant output voltage cannot prevent the load current flowing through the line from the regulator to the load center to cause an additional drop in voltage. This voltage drop is proportional to the changing load current, however the voltage regulator can be adjusted to correct for this drop in voltage between the regulator and load center by an appropriately set line drop compensator circuit. A current proportional to the line current is circulated through the line drop compensator by means of a current transformer inserted into the load circuit.
The initiation of a raising control action and a lowering control action, are spaced apart by a voltage zone which is known as the voltage bandwidth. This bandwidth must be higher than the minimum correction obtainable through the regulator, or hunting of the regulator will result. Furthermore, hunting of the regulator is limited by the time delay circuit which purpose is to withhold a tap-changing signal for a certain period of time even when the output voltage is outside of the voltage limit. The delay time is usually adjustable and may be set in intervals of seconds in a range of 10 to 120 seconds.
The conventional analog control devices may have additional features such as voltage reduction to cause the set voltage point to be lowered, or limiting the maximum output voltage for protecting users near the transformer, or measures for operation under reverse flow condition. Optional features usually require additional circuitry.
Such conventional analog control devices have been proven reliable and cost effective for a plurality of applications, however, since any analog circuit just can serve a special purpose, desirable optional features of such a control unit necessitate additional circuitry which more often compel a complete redesign of the basic unit. Therefore, conventional analog control units very often are constructed to fulfill just basic requirements. This may be illustrated by means of some examples. Even though line voltage and current information is made available in the control unit necessarily, such data is not accessable to an operator without additional circuitry or metering equipment. With line current and voltage information available, other power system data could be derived including real power, reactive power, and power factor. The conventional analog control unit does not include the means to easily visualize such parameters. Furthermore, analog control units are not general purpose units, they are designed for a specific purpose and not easily adjustable for different applications. This shows that the conventional analog control units have limitations and disadvantages.
For this reason, more recently digital control units including a programable processing unit have been proposed. One of the major advantages of digital control units is that such units can be designed more generically. Digital control devices may include basic features which can be utilized in a variety of applications for sensing, measuring and processing of a plurality of electrical parameters within power distribution systems and can provide a complete series of tools for monitoring and controlling the performance of a power distribution system. The generalized hardware structure of such a control unit is a powerful means for lowering the cost for many applications without requiring a redesign and even more expensive special purpose devices may become superfluous.
Such a digital control unit has been disclosed in a conference paper "Microprocessor-Based Control of Transformer Tap Changing" presented by J. Jindrick and N. Nohria at the Pacific Coast Electrical Association Engineering and Operating Conference, San Francisco, March 1982. A reprint was published by McGraw-Edison Company, Pittsburgh, Pa. The article outlines concepts of possible approaches for a digital control unit based upon either a single-chip micro-computer, a single-board computer, or a bus-oriented computer system and evaluates such different design concepts. The single-chip micro computer approach is believed to have limited flexibility and is, therefore, not to favored despite the lowest possible cost. On the other hand, the bus-oriented computer approach having various functional modules is believed to provide a virtually unlimited amount of flexibility with the constraint of higher cost. Accordingly, from the authors point of view the single-board computer concept has been selected as the most feasible design concept. The control unit includes a data-acquisition section which provides a data base for the microprocessor calculating desired power system information. The data acquisition section includes a 12 bit successive approximation analog-to-digital converter which is used to translate analog line voltage and current values obtained from the distribution system into digital information. A sample-and-hold circuit freezes the data during conversion. The obtained samples are then processed using a fast fourier transform (FFT) to determine the real and imaginary values of an input signal. An 8-channel multiplexer is provided to select up to eight channels of analog information which is scaled to values the multiplexer, the sample-and-hold circuit and the analog-to-digital converter can accept. Also, a timer is included which determines the precise instant when new data is to be acquired.
A number of other inputs and outputs to the computer are implemented including outputs to the tap-changing mechanism for use with either electric mechanical relays or solid-state relays. There are also provisions for inputs and outputs to other utility equipment such as an over/under frequency output for breaker or switch control in cogeneration applications.
The key to the flexibility of the proposed digital control unit is the ability of the microprocessor system to multiplex its hardware resources among several tasks. From the authors point of view, it is necessary to supply sophisticated software to support basic system operations, such as input/output timing, communications and data management. This software includes a data acquisition module governing the sampling of three analog input signals such as output voltage, output current and the differential voltage between input and output of the regulating transformer. The core of the software is a regulating module which processes all input information from every source to determine corresponding output signal. The input information to this module includes the described analog information and control parameters, such as rated voltage, bandwidth, line-drop compensation and time-delay values. An output software module supplies the proper signals to start a tap change; this module may also monitors the motor current to determine that the tap change in fact took place. Various other software modules are included for calculating desired power system information and from the authors point of view the software of a microprocessor regulating control will continue to evolve as additional features are defined and added to the control. Therefore, it is believed that the module approach is the most feasible approach to easily add new features.
While the proposed digital control represents a first step for replacing conventional analog electro-mechanical control units by programmable units in order to obtain a higher flexibility, further steps have to be taken to make the most efficient use of this digital technology. The flexibility of a microprocessor based control unit is but one advantage; certainly the cost factor is also of importance, but considering the remote installation of such control unit reliability is the most decisive factor. For this reason, a virtually unlimited amount of flexibility is less important than a simple, dependable hardware structure aiming at straight forward solutions of design objectives and still offering possibilities for including optional features which may enhance the power of such a tool. It may be emphasized that in contrast to quite a variety of applications of sophisticated digital equipment which accordingly needs more and more maintenance, digital equipment for this group of applications in rugged surroundings is required to be dependable in a wide temperature range almost without any maintenance. Furthermore, such digital equipment for use with power distribution systems should have at least some self-testing capabilities for elimiting basic souces of error which could cause the power distribution system to run out of control.